For modelling mechanisms controlling the activity of aluminium in soil solution of Japanese volcanic ash soils, known to contain abundant short-range ordered aluminiumsilicates (allophane) and soil organic matter (SOM), WHAM-Model V, a model of cation binding by humic substances, coupled with ion speciation code, has been applied. Soils that were previously characterized and extensively investigated by means of acid-base titrations and kinetic study conducted with speciation technique for dissolved Al species, in addition to mineralogical investigations, were used for this purpose.

To take account anion adsorption on positively charged surfaces on humic substances and clay minerals, surface charges estimated by observed anion adsorption were used as input parameters. Two parameters in Model V, concentrations of fulvic acid (FA) and humic acid (HA), have been optimized by fitting simulation output for pH and Al³⁺ activity in the acid-base titrations at 8 ºC to those observed.

In A and AB horizons, simulated pH and Al³⁺ activity fitted well to those observed upon optimization of FA and HA concentrations in Model V. Sum of optimized FA and HA concentrations were well correlated with the concentration of total soil organic carbon (SOC). Using optimized parameters, the changes in pH and Al³⁺ activity occurred after addition of 2 mM AlCl₃ were predicted by the WHAM-Model V.

In a B horizon, containing abundant allophane and less SOM, model prediction for the changes in Al³⁺ activity upon addition of AlCl₃ resulted in rather greater Al³⁺ activity at any predicted pH, compared with those observed. By incorporating a sub-model of allophane solubility (log K⁰_so = 6.7 (25 ºC)) into WHAM-Model V to take account equilibrium with allophane, the model predicted those changes in pH and Al³⁺ activity fairly well. This supports findings in our previous studies that, 1) in a B horizon containing abundant allophane and less SOM, solubility of allophane controls the activity of Al³⁺, 2) in A and AB horizons containing large amount of accumulated SOM, cation binding by humic substances controls Al³⁺ activity, despite considerable amount of Al and silica extractable by acid-oxalate are present in these horizons. These findings provide a basis toward a full-mechanistic modelling for Al³⁺ activity in volcanic ash soils.